1. Field of the Invention
The present invention relates to a switching system for a digital transmission network having at least one normal channel comprising, on the transmission side, means for applying, to the input of an emergency channel, the signal applied to the input of the normal channel and, on the reception side, means sensitive to the time shift between the signal of the normal channel and the signal of the emergency channel, adapted so as to control means for compensating said shift and for controlling means for switching from the normal channel to the emergency channel when said shift is compensated for.
Such a system is used for example in short wave link digital transmissions where, because of unfavorable propagation conditions or because of an equipment breakdown, a transmission channel has to be backed up. Taking into account the rate of appearance of this kind of difficulty and so that the cost of the system remains reasonable a single emergency channel is provided for several normal channels. It is therefore necessary to provide a switching system for replacing any normal channel by the emergency channel. In addition, it is preferable to effect such switching after making sure that these two channels have the same total propagation time so that the user does not notice the switching, which would be inevitably the case if such switching were effected without particular precautions.
2. Description of the Prior Art
Such systems are already known, described for example in patents U.S. Pat. Nos. 4,477,895 and 4,234,956. However, these systems are provided for operating with binary digital signals and cannot process bipolar digital signals, for example of the type coded according to the high density bipolar 3 code (HDB 3). Now this type of code is very well adapted to the transmission of the basic band digital signals over the great cable length met with in transmission and reception stations.
In fact a code HDBn corresponds to a signal at 3 levels, +1,0 and -1, which has no DC component and the code is provided so that more than n consecutive zeros are never met.
It is therefore interesting to have a switching system provided for operating with bipolar signals.
Such a switching system is already known, described in the application FR No. 2,359,553. This system comprises, on the emission side, means for applying to the input of an emergency channel a bipolar signal applied to the input of a normal channel and, on the reception side, means adapted for receiving bipolar signals and for controlling means, adapted for receiving bipolar signals, for switching from the normal channel to the emergency channel. However, no means sensitive to the time shift are provided nor means for compensating such a time shift. This system is therefore not provided for avoiding the discontinuities in the signal received by the user, which occur at the time of switching from the normal channel to the emergency channel when these two channels have different propagation times.
Another switching system is also known in which the input signal of said application means, on the transmission side, as well as the output signal from said switching means, on the reception side, are bipolar digital signals HDB3, and which is provided for switching without discontinuity in the case where the normal channel and the emergency channel have different propagation times.
In this system, said application means, on the transmission side, comprise a junction having an HDB3 input followed by a circuit for processing the HDB3 signal, comprising an amplifier with automatic gain control, a clock recovery circuit and a circuit for switching the positive and negative pulses. This processing circuit, or HDB3/binary+clock transcoder delivers on the one hand the clock signal and on the other two binary signals, i.e. at two levels 0 and 1, called respectively:
HDB.sup.+ signal (succession of the positive pulses of the HDB3), PA1 HDB.sup.- signal (succession of the rectified negative pulses of the HDB3 signal).
The HDB3/binary+clock transcoder is followed by another processing circuit which transforms the two signals HDB.sup.+ and HDB.sup.- into a single binary signal which, accompanied by the clock signal, is processed by conventional logic circuits for generating the two binary output signals of the junction, one of which is applied to the input of the normal channel and the other of which may be applied to the input of the emergency channel if the normal channel must be replaced.
Similarly, the two binary signals received on the channel to be replaced and on the emergecy channel are applied to the binary inputs of the means sensitive to the time shift and of the means for compensating the time shift. Using conventional logic circuits, they are compared and delayed with respect to each other so as to compensate for their time shift. When this compensation is achieved, the binary signal of the emergency channel, by means of logic gates, is sent in the place of the binary signal of the channel to be replaced, through a transcoder which transforms it into an HDB3 code signal on an output cable.
Thus, in this known system, the junction has an HDB3 input and two binary outputs, whereas the reception means have two binary inputs and one HDB3 output.
That limits the use of the known system to binary data transmission networks.
In addition, since binary signals do not propagate well over large cable lengths, the junction must be placed very close to the transmitter and the switching circuit very close to the receiver.
But this results in two disadvantages.
On the one hand, the length of channel likely to be replaced by the emergency channel is reduced, as well as the number of elements which it includes and the risks of final breakdown in a channel are increased.
On the other hand, the junction and the switching circuit must be materially located on the towers of the short wave station, for example.
The present invention overcomes these disadvantages.